 But Columbia University volcanologists found that the eruption of Costa Rica's Irazú Volcano in 1963 was likely triggered by magma that took a nonstop route from the mantle over just a few months.The Volcanic Explosivity Index (commonly shortened to VEI) is a scale, from 0 to 8, for measuring the strength of eruptions.It is used by the Smithsonian Institution's Global Volcanism Program in assessing the impact of historic and prehistoric lava flows.